Improved trivalent chromium-containing composition for aluminum and aluminum alloys

ABSTRACT

A low sludge trivalent chromium based conversion coating bath is provided which forms corrosion resistant coatings on aluminum and aluminum alloys by contact with the bath, which is suitable for use as a rinseable or dry-in place conversion coating for metal, that comprises trivalent chromium cations, fluorometallate anions, guanidinium ions and at least one organic anti-corrosion additive, and other optional components, as well as methods of making and using same, and metals coated using the compositions, the coated aluminum serving as an effective base for paint primers.

FIELD OF THE INVENTION

This invention relates to a method of treating metal surfaces to enhancecorrosion resistance and paint bonding characteristics and relates totrivalent chromium-containing compositions for coating aluminum andaluminum alloys used in such processes. More particularly, thisinvention relates to an aqueous composition, suitable for use as arinseable or dry-in place conversion coating for metal, that comprisestrivalent chromium cations, fluorometallate anions, their correspondingcounter-ions, and at least one organic anti-corrosion additive, andother optional components, and methods of making and using same, as wellas metals coated using the compositions.

BACKGROUND OF THE INVENTION

It is generally known to treat the surfaces of metals, such as zinc,cadmium, or aluminum with aqueous solutions that include hexavalentchromium, which contain chemicals that dissolve the surface of the metaland form insoluble films known as “chromate conversion coatings.” Thesehexavalent chromium-containing coatings are corrosion resistant andprotect the metal from various elements which cause corrosion. Inaddition, it is known that chromate conversion coatings generally havegood paint bonding characteristics and, therefore, provide an excellentbase for paint or other finishes.

Although the aforementioned coatings enhance corrosion resistance andpaint bonding properties, the coatings have a serious drawback, i.e.,the hazardous nature of hexavalent chromium. This is a serious problemfrom two viewpoints, one being the handling of the solution by operatorsand the other, the disposal of the used solution. Therefore, it ishighly desirable to have coating solutions and coatings that aresubstantially free of hexavalent chromium, but at the same time capableof imparting corrosion resistance and paint bonding properties which arecomparable to those imparted by conventional hexavalentchromium-containing coatings.

Of particular interest is the use of chromate conversion coatings onaircraft aluminum alloys due to the need for excellent corrosionresistance and the ability to serve as an effective base for paint.Conventional baths used to develop these coatings contain hexavalentchromium, and residual chromates in the coating are largely responsiblefor the high degree of corrosion inhibition. However, these samechromates are hazardous and their presence in waste water effluents isseverely restricted. It would therefore, be desirable to provide acoating for aluminum and its alloys, and for sealing of anodizedaluminum, utilizing other chemicals as an alternative to the hexavalentchromate coatings.

Trivalent chromium has been used in conversion coatings in addition toand instead of hexavalent chromium in an attempt to produce replacementsfor hexavalent chromium-containing coatings, but to date, these attemptshave been only somewhat successful. Particularly in the aerospaceindustry, there remains a need for improved performance in trivalentchromium containing coating compositions. When applied to thoroughlydeoxidized aluminum alloys, in particular AA2024T3, trivalent chromiumcontaining coating compositions result in coatings with reducedcorrosion performance as compared to chromate coatings. Another drawbackof current trivalent chromium containing coatings is that they aredifficult to detect with the naked human eye. This presents problems indifferentiating coated and uncoated substrates and in performing qualitychecks for coating coverage.

Another drawback of some trivalent chromium corrosion preventivecompositions is that they are not storage stable in that, upon aging forabout 1-2 weeks, a precipitate begins to form, even in unusedcompositions. In use, the working bath can generate a significant amountof sludge that must be removed, which can result in costly down-time forthe processing line and disposal issues, or left undisturbed duringprocessing to avoid powder residue on the coated substrate. The onset ofprecipitation in the bath also has a negative impact on the conversioncoating formed. The conversion coatings from aged baths of the prior arthave reduced corrosion resistance. Thus there is a need for an improvedtrivalent chromium corrosion preventive coating that overcomes these andother drawbacks of the prior art.

SUMMARY OF THE INVENTION

It is an aspect of the invention to provide compositions for treating ametal surface comprising a component of fluorometallate anions; a sourceof chromium(III) cations; a source of guanidinium cation; a source ofnitrate; at least one organic anti-corrosion additive and, optionally,one or more of the following components: a component of free fluorideions; a component of surfactant molecules; a pH adjusting component anda viscosity increasing component.

It is an further aspect of the invention to provide compositions fortreating a metal surface comprising a source of anions at least oneelement “M” selected from the group consisting of titanium, zirconium,hafnium, silicon, aluminum, and boron; a source of chromium(III)cations; a source of guanidinium cation; a source of nitrate; acomponent of free fluoride ions; and, optionally, at least one organicanti-corrosion additive; one or more of the following components: acomponent of surfactant molecules; a pH adjusting component and aviscosity increasing component.

It is at least one aspect of this invention to provide a novelchromium-containing solution for treating aluminum, which contains nohexavalent chromium. It is another aspect of this invention to provide acomposition for treating aluminum which essentially contains chromiumonly in its trivalent oxidation state.

It is still another aspect of this invention to provide a trivalentchromium-containing solution wherein said chromium has little or notendency to precipitate from the solution upon storage at 60 degrees C.for at least, in increasing order of preference, 2 weeks, 4 weeks, 6weeks, 8 weeks, 10 weeks or 12 weeks.

It is likewise an aspect of the invention to provide a coating, that isformed by contacting the metal surface with a composition of theinvention and then rinsing or drying-in-place, which comprises chromiumin substantially only trivalent form and which provides salt sprayresistance of at least, with increasing preference, 96, 120, 144, 168,192, 216, 240, 264, 288, 312, 336, 360, 408, 456, 480, 504 hoursaccording to ASTM B-117 (03).

This invention is particularly useful for forming a adherent corrosionresistant conversion coating on metal surfaces consisting predominantlyof steel, titanium and its alloys, aluminum and its alloys, magnesiumand its alloys and/or zinc and its alloys; such substrates includealuminum/zinc alloy coated steel and iron/zinc alloy coated steel, e.g.Galvalume™ and Galvaneal™. One of skill in the art will understand“predominantly” as used herein to mean the predominant element is theone comprising the greatest amount by weight of the alloy. Other objectswill be apparent to those skilled in the art from the description below.

Except in the stated aspects and the operating examples, or whereotherwise expressly indicated, all numerical quantities in thisdescription indicating amounts of material or conditions of reactionand/or use are to be understood as modified by the word “about” indescribing the broadest scope of the invention. Practice within thenumerical limits stated is generally preferred. Numerical rangesprovided throughout the description and claims are meant to include allsubset ranges, that is, it is intended that the range comprises all subranges found within the stated range, for example C₁₋₁₀ also disclosesC₂₋₁₀, C₁₋₉ and C₃₋₇, and an amount of 1-100 also discloses 1-99, 2-100,and 45-50. Also, throughout this description, unless expressly stated tothe contrary: percent, “parts of”, and ratio values are by weight; theterm “fluorometallate anions” as used herein includes anions containingfluorine and at least one of a transition metal and a metalloid; thedescription of a group or class of materials as suitable or preferredfor a given purpose in connection with the invention implies thatmixtures of any two or more of the members of the group or class areequally suitable or preferred; description of constituents in chemicalterms refers to the constituents at the time of addition to anycombination specified in the description or of generation in situ bychemical reactions specified in the description, and does notnecessarily preclude other chemical interactions among the constituentsof a mixture once mixed; specification of materials in ionic formadditionally implies the presence of sufficient counter ions to produceelectrical neutrality for the composition as a whole (any counter ionsthus implicitly specified should preferably be selected from among otherconstituents explicitly specified in ionic form, to the extent possible;otherwise such counter ions may be freely selected, except for avoidingcounter ions that act adversely to the objects of the invention); theterm “paint” includes all like materials that may be designated by morespecialized terms such as primer, lacquer, enamel, varnish, shellac,topcoat, and the like; and the term “mole” and its variations may beapplied to elemental, ionic, and any other chemical species defined bynumber and type of atoms present, as well as to compounds with welldefined molecules.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

This invention relates to an aqueous acidic composition, suitable foruse as a rinseable or dry-in place conversion coating for metal, thatcomprises trivalent chromium cations, fluorometallate anions, theircorresponding counter-ions, and at least one organic anti-corrosionadditive, and other optional components, and methods of making and usingsame, as well as metals coated using the compositions. The inventionalso relates to an aqueous acidic composition, suitable for use as arinseable or dry-in place conversion coating for metal, comprising asource of anions at least one element “M” selected from the groupconsisting of titanium, zirconium, hafnium, silicon, aluminum, andboron; a source of chromium(III) cations; a source of guanidiniumcation; a source of nitrate; a component of free fluoride ions; and,optionally, at least one organic anti-corrosion additive; one or more ofthe following components: a component of surfactant molecules; a pHadjusting component and a viscosity increasing component.

One embodiment of the present invention provides a liquid conversioncoating composition that comprises, preferably consists essentially of,or more preferably consists of, water and:

-   -   (A) a source of fluorometallate anions, each of said anions        consisting of:        -   (i) at least four fluorine atoms; and        -   (ii) at least one atom of an element “M” selected from the            group consisting of titanium, zirconium, hafnium, silicon,            aluminum, and boron; and, optionally, one or both of        -   (iii) at least one ionizable hydrogen atom; and        -   (iv) at least one oxygen atom;    -   (B) a source of chromium(III) cations;        wherein the molar ratio of chromium(III) cations to        component (A) ranges from 20:1 to 50:1.    -   (C) a source of guanidinium cation; and, optionally, one or more        of the following components:    -   (D) a source of nitrate anions;    -   (E) an organic corrosion inhibitor, preferably a triazole, for        example a tolyltriazole that is not part of any of immediately        previously recited components (A) through (D);    -   (F) a component of free fluoride ions that are not part of any        of immediately previously recited components (A) through (E);    -   (G) a component of surfactant molecules that are not part of any        of immediately previously recited components (A) through (F);    -   (H) a pH adjusting component that is not part of any of the        immediately previously recited components (A) through (G); and    -   (I) a viscosity increasing component that is not part of any of        the immediately previously recited components (A) through (H).        It should be understood that the components and sources listed        need not necessarily all be provided by separate chemicals.

In another embodiment, the present invention provides a liquidconversion coating composition that comprises, preferably consistsessentially of, or more preferably consists of, water and:

-   -   (A) a source of ions of at least one element “M” selected from        the group consisting of titanium, zirconium, hafnium, silicon,        aluminum, and boron;    -   (B) a source of chromium(III) cations;        wherein the molar ratio of chromium(III) cations to element “M”        ranges from 20:1 to 50:1;    -   (C) a source of guanidinium cation; and, optionally, one or more        of the following components:    -   (D) a source of nitrate anions;    -   (E) an organic corrosion inhibitor, preferably a triazole, for        example a tolyltriazole that is not part of any of immediately        previously recited components (A) through (D);    -   (F) a component of free fluoride ions that are not part of any        of immediately previously recited components (A) through (E);    -   (G) a component of surfactant molecules that are not part of any        of immediately previously recited components (A) through (F);    -   (H) a pH adjusting component that is not part of any of the        immediately previously recited components (A) through (G); and    -   (I) a viscosity increasing component that is not part of any of        the immediately previously recited components (A) through (H).        It should be understood that the components listed need not        necessarily all be provided by separate chemicals.

In one embodiment, the source of ions of at least one element “M”selected from the group consisting of titanium, zirconium, hafnium,silicon, aluminum, and boron is selected from fluorometallates,carbonates, basic carbonates, element “M” in the zero oxidation state,oxide, metal hydroxides and the like which are soluble in thecomposition and wherein the counter ions to the element “M” do notinterfere with the objects of the invention.

In another embodiment, the present invention provides a method ofcoating metal substrates, in particular substrates having surfaces ofaluminum and alloys of aluminum:

-   -   (I) contacting a metal substrate surface with the above        described composition of the invention to form a wet coated        metal substrate surface;    -   (II) optionally, rinsing the wet coated metal substrate surface;        and    -   (III) drying the wet coated metal substrate surface, optionally        with heating.

The compositions of the invention have been developed as hexavalentchromium-free. Although not preferred, formulations according to theinvention can be made including hexavalent chromium, in which case thesecompositions having a non-zero amount of hexavalent chromium desirablycontain less than 0.04, 0.02, 0.01, 0.001, 0.0001, 0.00001, or 0.000001percent by weight of hexavalent chromium. The amount of hexavalentchromium present in the compositions of the invention is desirablyminimized and preferably only trace amounts are present, most preferablyno hexavalent chromium is present. In a preferred embodiment of theinvention, the composition and the resulting coating are substantiallyfree, desirably essentially free, of hexavalent chromium.

It is known in the prior art to oxidize some of the trivalent chromiumin a coating to form hexavalent chromium, see U.S. Pat. No. 5,304,257.In the present invention, it is desirable that the coatings formed bycompositions according to the invention contain little or no hexavalentchromium, though not preferred the compositions may contain hexavalentchromium in the amounts as recited in the immediately precedingparagraph. It will be understood by those of skill in the art that theinvention includes coatings that contain no hexavalent chromium butwhich may, due to subsequent exposure to weathering or other treatments,contain hexavalent chromium resulting from oxidation of the trivalentchromium in the coating.

Various embodiments of the invention include processes for treatingsurfaces as described above, optionally in combination with otherprocess steps, such as cleaning, deoxidizing, rinsing, and subsequentfurther protective coatings over those formed according to theinvention; compositions useful for treating surfaces as described above;and articles of manufacture including surfaces treated according to aprocess of the invention.

Independently of the concentration of Component (A), the fluorometallateanions preferably are fluorosilicate (i.e., SiF₆ ⁻²), fluorotitanate(i.e., TiF₆ ⁻²) or fluorozirconate (i.e., ZrF₆ ⁻²), more preferablyfluorotitanate or fluorozirconate, most preferably fluorozirconate. Aworking composition for use in a process according to this inventionpreferably has a concentration of fluorometallate anions of at least,with increasing preference in the order given, 0.4, 0.5, 0.6, 0.7, 0.8,0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2,2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6,3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9 or 5.0grams per liter of total working composition. The upper limit offluorometallate anions is generally based upon solubility and/or themolar ratio of chromium(III) cations to component (A), and preferably isnot more than with increasing preference 15, 14.5, 14, 13.5, 13, 12.5,12, 11.5, 11, 10.5 or 10.0 grams per liter of total working composition.The cation for the fluorometallate anion may be selected from ions ofGroup IA elements, or ammonium ions. Preferably the cation is K or H,most preferably H.

Component (B) as defined above is to be understood as including one ormore of the following sources of trivalent chromium cations: nitrates,sulfates, and fluorides of chromium(III). In a preferred embodiment,Component (B) comprises, preferably consists essentially of, mostpreferably consists of trivalent chromium fluoride. The totalconcentration of the trivalent chromium cation source in a workingcomposition according to the invention is preferably at least, withincreasing preference in the order given, 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 21, 22,23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34 or 35 g/l, andindependently, primarily for reasons of economy and solubility, ispreferably not more than, with increasing preference in the order given,100, 90, 80, 70, 60, 55, 50, 45, 44, 43, 42, 41, 40, 39, 38, 37 or 36g/l. The source of trivalent chromium is selected for solubility in theworking bath, which is often a function of the nature and amounts ofother components in the bath.

Independent of the amount of components (A) and (B) it is desirable thatthe molar ratio of trivalent chromium cations to the element “M”,regardless of source, but also specifically including when the source of“M” is one of the fluorometallate anions, i.e. titanium, zirconium,hafnium, silicon, aluminum, and boron, is within the range of 1.27:1 to68.4:1. Preferred ratios are at least, with increasing preference in theorder given, 1.75:1, 3.5:1, 5.25:1, 7.0:1, 8.77:1, 10.5:1, 12.3:1,14.0:1, 15.8:1, or 17.5:1 and preferably not more than, with increasingpreference in the order given, 52.6:1, 50.9:1, 47.4:1, 43.8:1.

Where “M” is zirconium, and independent of the amount of components (A)and (B) it is desirable that the weight ratio of trivalent chromiumcations to the element “M”, is within the range of 0.725:1 to 39:1.Preferred ratios are at least, with increasing preference in the ordergiven, 1:1, 2:1, 3:1, 4:1, 5: 1, 6: 1, 7:1, 8:1, 9:1 or 10:1 andpreferably not more than, with increasing preference in the order given,30:1, 29:1, 27:1 or 25:1.

Component (C), as defined above is to be understood as including one ormore of the following sources of guanidinium cation: acetates,aminosulfonates, carbonates, nitrates, sulfates, sulfamates andthiocyanates. Component (C) is generally soluble in water and may beselected from any guanidine containing compound that providesguanidinium cation in solution and does not otherwise interfere with thecorrosion resistance and adhesion of coatings according to theinvention.

In a preferred embodiment, Component (C) comprises, preferably consistsessentially of, most preferably consists of guanidine nitrate is thesource of the guanidinium cation. Guanidine nitrate may be provided as asupplied material, or formed via a reaction of other materials, such asguanidine carbonate and nitric acid, and other components that can reactto produce, or otherwise form, guanidine nitrate.

The amount of component (C), useful in compositions according to theinvention is a quantity that provides an amount of guanidinium cationequal to guanidinium cation resulting from guanidine nitrate beingpresent in a working composition at concentrations of at least, withincreasing preference in the order given, 0.25, 0.5, 1, 1.5, 2, 2.5, 3,3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.512, 12.5, 13, 13.5, 14, 14.5, or 15 g/l, and independently, primarilyfor reasons of economy and solubility, is preferably not more than, withincreasing preference in the order given, 50, 45, 40, 35, 30, 28, 26,24, 22, 20, 18 or 16 g/l.

Independent of the amount of components (A) and (C) it is desirable thatthe weight ratio of guanidinium cation to element “M” in thefluorometallate anions, i.e. titanium, zirconium, hafnium, silicon,aluminum, and boron, is within the range of 0.5:1 to 20:1. Preferredratios are at least, with increasing preference in the order given,0.6:1, 0.75:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4,5:1 or 5:1,and preferably not more than, with increasing preference in the ordergiven, 20:1, 17.5:1, 15:1, 12.5:1, 10:1 or 7.5:1.

Component (D) the source of nitrate anions is generally soluble in waterand may be selected from any nitrate source where the counter cationdoes not otherwise interfere with the corrosion resistance and adhesionof coatings according to the invention. Component (D) as defined aboveis to be understood as including one or more of the followingwater-soluble sources of nitrate anions: nitric acid, nitrate salts ofalkali metals, alkaline earth metals, transition metals, lanthanide andactinide series elements, as well as guanidine nitrate. The nitrateanion may be present in a range of 0.25 g/1 up to the solubility limitof the source of nitrate anion. Desirably, nitrate anion is present in arange of 0.5 to 25 g/1, preferably 1-10 g/1. In a preferred embodiment,Component (D) comprises, preferably consists essentially of, mostpreferably consists of guanidine nitrate. Preferred transition metalnitrate salts include those of Ti, Zr, Cr and Co.

Component (E) is a water-soluble or water-miscible organic corrosioninhibitor. Component (E) as defined above is to be understood be basedupon one or more of the following: a thiazole compound, a triazolecompound, a tetrazole compound and an imidazole compound. Thesecompounds may be themselves soluble or miscible in water or the moresoluble salts of said compounds may be used.

In a preferred embodiment, Component (E) comprises, preferably consistsessentially of, most preferably consists of a triazole, and in at leastone embodiment is tolyltriazole. Other triazoles include, but are notnecessarily limited to, benzotriazole, carboxytriazole, sodium triazole,sodium tolyltriazole, potassium tolytrazole and mercaptobenzotriazole,to name a few. If present, component (E) is present in the workingcomposition in at least, with increasing preference in the order given,0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1.0, 1.25, 1.4, 1.5,1.75, 2.0, 2.25, 2.5, 2.75 or 3.0 g/l, and independently, primarily forreasons of economy and solubility, is preferably not more than, withincreasing preference in the order given, 10, 9, 8, 7, 6, 5.5, 5, 4.5,4.25, 4.0, 3.8, 3.7 or 3.5 g/l.

Independent of the amount of components (A) and (E) it is desirable thatthe weight ratio of organic corrosion inhibitor to the element “M” inthe fluorometallate anions, i.e. titanium, zirconium, hafnium, silicon,aluminum, and boron, is within the range of 0.03:1 to 6.0:1. Preferredratios are at least, with increasing preference in the order given,0.03:1, 0.075:1, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.50:1, .075:1, 1:1,1.25:1, 1.5:1, 1.75:1 or 2:1 and preferably not more than, withincreasing preference in the order given 5.75:1, 5.5:1, 5:1, 4.5:1, 4:1,3.5:1, 3:1 or 2.5:1.

A component of free fluoride ions (F) may optionally be provided, whichmay or may not be part of any of immediately previously recitedcomponents (A) through (E). This component may be supplied to thecomposition by hydrofluoric acid or any of its partially or completelyneutralized salts that are sufficiently water soluble. At least foreconomy, component (F) is preferably supplied by aqueous hydrofluoricacid, and independently preferably is present in a concentration that isat least, with increasing preference in the order given, 0.10, 0.30,0.50, 0.60, 0.70, 0.80, or 0.90 ppt of its stoichiometric equivalent asHF. Independently, in a working composition to be used in a processaccording to the invention, the concentration of component (F), measuredas its stoichiometric equivalent as HF, preferably is not more than,with increasing preference in the order given, 10, 8.0, 6.0, 4.0, 3.0,2.0, 1.5, 1.3, or 1.1 ppt. Suitable sources of free fluoride ions areknown to those of skill in the art. Preferably, the source of (F) is HF.

Component (G), if used, is chosen from surfactants that are stable inthe working bath pH and concentration of fluorine. Suitable surfactantscan be anionic surfactants, such as salts of carboxylic acids,alkylsulphonates, alkyl-substituted phenylsulphonates; nonionicsurfactants, such as alkyl-substituted diphenylacetylenic alcohols,fluorosurfactants and nonylphenol polyoxyethylenes; and cationicsurfactants such as alkylammonium salts; all of these may and preferablydo contain fluorine atoms bonded directly to carbon atoms in theirmolecules. Each molecule of a surfactant used preferably contains ahydrophobe portion that (i) is bonded by a continuous chain and/or ringof covalent bonds; (ii) contains a number of carbon atoms that is atleast, with increasing preference in the order given, 10, 12, 14, or 16and independently preferably is not more than, with increasingpreference in the order given, 30, 26, 22, or 20; and (iii) contains noother atoms, in addition to carbon, except hydrogen, halogen, andether-bonded oxygen atoms. Component (G) is most preferably a non-ionicfluorosurfactant, such materials are known in the art and commerciallyavailable under the Fluorad® trade name from 3M Company, under theZonyl® trade name from E.I. du Pont de Nemours and Company, under theMasurf® trade name from Mason Chemical Co and under the APFS trade namefrom Advanced Polymer Inc.

[0036.] A working composition according to the invention may contain,with increasing preference in the order given, at least 0.010, 0.030,0.050, 0.070, 0.080, 0.090, or 0.100 ppt of component (G) andindependently preferably, primarily for reasons of economy, contains notmore than, with increasing preference in the order given, 5.0, 2.5,1.30, 0.80, 0.60, 0.40, 0.30, 0.20, 0.18, 0.15, 0.13, or 0.11 ppt ofcomponent (G).

Compositions according to the invention are acidic. The pH preferably isat least, with increasing preference in the order given, 2.0, 2.10,2.30, 2.50, 2.70, 2.90, 3.0, 3.10, 3.20, 3.30, 3.40, 3.50, 3.60, 3.70,3.80, 3.90, or 4.0 and independently preferably is not more than, withincreasing preference in the order given, 7.0, 6.50, 6.0, 5.50, 4.75,4.50, 4.30 or 4.20. A pH adjusting component (H), which may or may notbe part of any of the immediately previously recited components (A)through (G) can be added to the composition in an amount sufficient toproduce a pH in the above-recited range, as necessary. A pH adjustingcomponent may be any acid or a base, known in the art which does notinterfere with the objects of the invention. In one embodiment, the pHadjuster is an acid, such as sulfuric acid, phosphoric acid, nitrousacid or nitric acid, preferably nitrous acid or nitric acid. In anotherembodiment, the pH adjusting component comprises a base such as ammoniumhydroxide, KOH, or NaOH, and desirably is ammonium hydroxide.

Concentrates may be made, as single or multiple pack products, at 100 to400% of the working bath concentrations, described herein, provided thatthe higher concentration does not result in instability, such asprecipitation. Working baths can desirably be made by simple dilutionwith water, preferably deionized water. Generally, the ranges of amountsdisclosed herein relate to working baths for dried-in-placeapplications. Where compositions of the invention are to be used inprocesses where the coated metal substrate is rinsed after removal fromthe conversion coating working bath of the invention, then, if desired,at least for economy, the amounts of the bath components may be reducedto 75% by weight of that used for dried-in-place applications.

Replenisher compositions useful for replenishing working baths accordingto the invention are provided herein. Replenisher compositions maycomprise Components (A)-(D), as described above. (A) and (B) beingpresent in an amount of about 100-200% of the concentrations disclosedfor the working baths, and (D) and (E) being present in an amount of200-300% of the concentrations disclosed for the working baths.

A working composition according to the invention may be applied to ametal workpiece, rinsed and dried thereon by any convenient method,several of which will be readily apparent to those skilled in the art.For example, coating the metal with a liquid film may be accomplished byimmersing the surface in a container of the liquid composition, sprayingthe composition on the surface, coating the surface by passing itbetween upper and lower rollers with the lower roller immersed in acontainer of the liquid composition, contact with a brush or feltsaturated with the liquid treatment composition, and the like, or by amixture of methods. Excessive amounts of the liquid composition thatmight otherwise remain on the surface prior to drying may be removedbefore drying by any convenient method, such as rinsing, drainage underthe influence of gravity, passing between rolls, and the like.

The temperature during application of the liquid composition may be anytemperature within the liquid range of the composition, although forconvenience and economy in application, normal room temperature, i.e.,from 20-27° C., is usually preferred.

Application of compositions of the instant invention provide improvedadhesive bonding to subsequently applied protective layers, such aspaints, lacquers and other resin based coatings.

The total add-on mass (after drying) of the coating applied in a processaccording to the invention is at least, with increasing preference inthe order given, 10, 12.5, 15, 17.5, 20, 22.5, 25, 27.5, 30, 32.5, 35,37.5 or 40 milligrams per square foot of surface coated (hereinafterusually abbreviated as “mg/ft²”). Independently, at least equalcorrosion resistance ordinarily will be achieved even if the add-on massis not, and therefore for reasons of economy the add-on mass preferablyis not greater than, with increasing preference in the order given, 150,130, 120, 110, 90, 75, 60, 50 or 45 mg/ft².

The add-on mass of the protective film formed by a process according tothe invention may be conveniently monitored and controlled by measuringthe add-on weight or mass of the metal atoms in the anions of component(A) as defined above, or of chromium, except in the unusual instanceswhen the initial protective coating and/or the underlying metalsubstrate contains the same metal element(s). The amount of these metalatoms may be measured by any of several conventional analyticaltechniques known to those skilled in the art. The most reliablemeasurements generally involve dissolving the coating from a known areaof coated substrate and determining the content of the metal of interestin the resulting solution. The total add-on mass can then be calculatedfrom the known relationship between the amount of the metal in component(A) and the total mass of the part of the total composition that remainsafter drying. However, this method is often impractical for use withthis invention, because the area touched up is not always preciselydefined. A more practical alternative is generally provided by smallarea X-ray spectrographs that, after conventional calibration, directlymeasure the amount(s) per unit area of individual metallic element(s)present in a coating, free from almost all interferences except the sameelements present in other coatings on, or in a thin layer near thesurface of, the underlying metal surface itself.

The effectiveness of a treatment according to the invention appears todepend predominantly on the total amounts of the active ingredients thatare present on each unit area of the treated surface, and on the natureof the active ingredients and their ratios to one another, rather thanon the concentration of the acidic aqueous composition used, and thespeed of drying has not been observed to have any technical effect onthe invention, although it may well be important for economic reasons.If practical in view of the size of the object treated and the size ofthe areas of the object to be treated, drying may be speeded byplacement of the surface to be treated, either before or afterapplication to the surface of a liquid composition in a processaccording to the invention, in an oven, use of radiative or microwaveheating, or the like. Heating the surface before treatment is preferredover heating after treatment when practical, and prewarming temperaturesup to at least 65° C. may be satisfactorily used. If ample time isavailable at acceptable economic cost, a liquid film applied accordingto this invention often may simply be allowed to dry spontaneously inthe ambient atmosphere with equally good results insofar as theprotective quality of the coating is concerned. Suitable methods foreach circumstance will be readily apparent to those skilled in the art.

Preferably, the surface to be treated according to the invention isfirst cleaned of any contaminants, particularly organic contaminants andmetal fines and/or inclusions. Such cleaning may be accomplished bymethods known to those skilled in the art and adapted to the particulartype of substrate to be treated. For example, for galvanized steelsurfaces, the substrate is most preferably cleaned with a conventionalhot alkaline cleaner, then rinsed with hot water and dried. Foraluminum, the surface to be treated most preferably is first contactedwith a conventional water alkaline cleaner, then rinsed in warm water,then, optionally, contacted with a neutralizing acid rinse and/ordeoxidized, before being contacted with an acid aqueous composition asdescribed above. Deoxidizing may include mechanical or chemical removalof surface oxides from the metal surface to be coated. Desirably, atleast 50%, 75% or 100% of the surface oxides are removed from the metalsurface to be coated.

After the preparatory cleaning, the surface may be dried by absorptionof the cleaning fluid, evaporation, or any suitable method known tothose skilled in the art. Corrosion resistance is usually less thanoptimal when there is a delay between the preparatory cleaning, orcleaning and drying, and the coating of the surface. The time betweencleaning, or cleaning and drying, and coating the surface should be nomore than, in increasing order of preference, 48, 24, 12, 6.0, 5.0, 4.0,3.0, 2.0, 1.0, 0.50, 0.25, or 0.1 hours.

The practice of this invention may be further appreciated byconsideration of the following, non-limiting, working examples.

EXAMPLES Example 1

Samples of trivalent chromium conversion coating compositions were madewith various nitrate sources: guanidine nitrate, lanthanum nitrate, andmanganese nitrate. Guanidine nitrate and lanthanum nitrate containingsamples were tested at 0.5, 2, and 5 grams/liter in a base trivalentchromium conversion coating composition (base composition).

Sample preparation: One master batch of the base composition was madeand split into separate samples, one sample having no nitrate wasreserved as a control, and the other samples had different nitratesources added to make the nitrate containing samples. Manganese nitratewas tested at 1, 4, and 10 g/L because that material was tested as a 50%liquid, where the others were tested as 100% active solids.

The base composition for all of the Examples unless stated otherwisecomprised:

Material (%) DI water 97.2 Chromium Floride 2.3 Fluorozirconic acid(45%) 0.3 Ammonium hydroxide 0.2 Total 100%

Two panels of aluminum alloy AA2024-T3 were processed for each of thebaths in Table 1 were processed as follows: The panels were cleaned for10 minutes in the aqueous alkaline degreaser Turco 6849 at 20% in tapwater at a temperature of 57° C. The panels were rinsed in warm water(about 38° C.) overflowing tap water until the surface was water breakfree (about 3 minutes). Then the panels were deoxidized with TurcoDeoxalume 2310 (made up at 15% Deoxalume 2310 with 25% nitric acid) andrun for 5 minutes at room temperature (22° C.). Both Turco products arecommercially available from Henkel Corporation. After the deoxidizingstep, the panels were rinsed for 2 minutes in cool tap water (about 13°C.). From the rinse bath, the panels were immersed into one of theformulations in Table 1, and processed for 10 minutes. Lastly, thepanels were rinsed in deionized water for 3 minutes and hung to air dryat room temperature. The panels were allowed to cure for 24 hours atroom temperature, and then subjected to 336 hours ASTM B117 (03) saltfog corrosion testing.

The results are shown below on Table 1.

TABLE 1 Number of Pits per 3 × 8 inch panel Level of Nitrate inGrams/Liter 0 0.5 2 5 Panel 1 Panel 2 Panel 1 Panel 2 Panel 1 Panel 2Panel 1 Panel 2 base composition 50 50 base composition + 4 2 10 6 0 1guanidine nitrate base composition + 5 40 25 25 7 20 lanthanium nitratebase composition + 40 25 40 40 25 6 manganese nitrate

The results show that all of the guanidine nitrate samples hadparticularly improved corrosion resistance and specifically at 5 g/L.

Example 2

These examples test the performance of the invention on two additionaltypes of aluminum aerospace alloys (specifically AA6061-T6 andAA7075-T6). A new bath of the 5 g/l guanidine nitrate in basecomposition bath was made up. This new bath was tested alongside acontrol base composition and an older, well used bath that contained 5g/l guanidine nitrate in base composition.

Two panels each of aluminum alloy 2024-T3, AA6061-T6 and AA7075-T6 wereprocessed and salt fog corrosion test according to the process used inExample 1. The result can be seen in Table 2 below.

TABLE 2 Number of Pits per 3 × 8 inch panel Aluminum Alloy AA2024-T3AA6061-T6 AA7075-T6 Panel 1 Panel 2 Panel 1 Panel 2 Panel 1 Panel 2 BaseComposi- 25 25 5 3 0 5 tion Control Base Composi- 3 5 0 3 1 0 tion + 5g/L Guanidine nitrate - Old bath Base Composi- 0 0 0 1 0 0 tion + 5 g/LGuanidine nitrate - New bath

Example 3

In this example, the impact of different deoxidizers was studied. To dothis, a ladder study of Deoxidizer HX-357 was run on the guanidinemodified base composition bath and the base composition (unmodified).Deoxidizer HX-357 was studied at 0, 1, 2, 3, 4, & 5% concentrations indeionized water.

Two panels of aluminum alloy 2024-T3 were processed in each of thebaths. The panels were cleaned for 10 minutes in Turco 6849 at 20% intap water at a temperature of 57° C. The panels were rinsed in warm(about 38° C.) overflowing tap water until the surface was water breakfree (about 3 minutes). Then the panels were deoxidized with thedeoxidizer from the Deoxidizer HX-357 ladder study, and run for 5minutes at room temperature (about 22° C.). After deoxidizing, thepanels were rinsed for 2 minutes in cool tap water (about 13° C.). Allthe panels were then desmutted using Turco Liquid Smut Go NC at 20%concentration for 2 minutes. After desmutting, the panels were rinsedfor 2 minutes in cool tap water (about 13° C.). From the rinse bath, thepanels were immersed into either base composition (unmodified) or theguanidine modified base composition, and processed for 10 minutes.Lastly, the panels were rinsed off in deionized water for 3 minutes andhung to air dry at room temperature. The panels were allowed to cure for24 hours and subjected to 336 hours ASTM B117 salt fog corrosiontesting. The results are shown in Table 3 below.

TABLE 3 Number of Pits per 3 × 8 inch panel Base Composition BaseComposition + 5 g/L (unmodified) Guanidine nitrate 0% Concentration of200 0 Deoxidizer HX-357 1% Concentration of 0.5 0 Deoxidizer HX-357 2%Concentration of 10 8 Deoxidizer HX-357 3% Concentration of 15 10Deoxidizer HX-357 4% Concentration of 75 25 Deoxidizer HX-357 5%Concentration of 100 40 Deoxidizer HX-357

The results show that the addition of guanidine nitrate again improvesthe corrosion resistance of the control coating. In this test, the nodeoxidizer sample (0% Deoxidizer HX-357) and the 1% Deoxidizer HX-357show no corrosion pits at all on the panels after 336 hours salt fogexposure.

Example 4

In this example, a design-of-experiment (DOE) was run to test the amountof guanidine nitrate, and to confirm the statistical significance of theinvention. A mixture design was run to test the amount of guanidinenitrate and a second additive (manganese sulfate which provedunhelpful). A master batch of 20 g/L guanidine nitrate in the basecomposition was made, a master batch of 20 g/L manganese sulfate in thebase composition was made, and a master batch of the base compositionwas made for use in the mixture design. Various levels of each of thesethree samples were blended into 2 liter samples. These mixturestranslated into the guanidine nitrate concentration being varied between0 g/L and 10 g/L (two times the previous tests).

Two panels of aluminum alloy 2024-T3 were processed in each of the bathsaccording to the process and testing used in Example 1 for each one ofthe formulations shown in Table 4 below. After corrosion testing thepanels were rated for the number of pits per panel, with the best casebeing 0. The two panels were rated, averaged and input into the DesignExpert 7.1.1 DOE software. The results of the DOE analysis are shown inTable 4 below.

TABLE 4 Amount (in grams) of base compo- base compo- sition w 20 sitionw 20 Salt Spray base g/L Guanidine g/L Manganese Panel Panel compositionNitrate Sulfate 1 2 Run #1 666.7 666.7 666.7 12 5 Run #2 2000 0 0 3 10Run #3 0 2000 0 0 2 Run #4 1000 1000 0 3 0 Run #5 0 0 2000 3 10 Run #6333.3 1333.3 333.3 0 2 Run #7 0 1000 1000 7 11 Run #8 0 0 2000 13 10 Run#9 333.3 333.3 1333.3 8 20 Run #10 1000 1000 0 5 15 Run #11 1000 0 100016 2 Run #12 1333.3 333.3 333.3 6 7 Run #13 0 2000 0 10 0 Run #14 2000 00 27 5

This example again indicates that the addition of guanidine nitrate doesindeed improve the corrosion resistance of the base composition. And inthis example, a level of 10 g/L performed better than the original 5g/L. The DOE predicted the ratings improved from 9.6 to 2 pits per panelwith the guanidine compared to the control, where the actual observedvalues improve from 11.3 to 3—significant improvement in both cases.

Example 5

A master batch of the base composition was made and divided into four,2-liter samples. The samples were modified as follows:

-   -   Sample A was used as a control, no additives were added.    -   Sample B was the master batch with 10 grams of guanidine nitrate        added.    -   Sample C was the master batch with 0.2 grams of tolyltriazole        added.    -   Sample D was the master batch with 10 grams of guanidine nitrate        and 0.2 grams of tolyltriazole added.

To test the corrosion performance, the four samples were applied toseveral sets of AA2024-T3 bare aluminum panels. First, the panels werecleaned with Turco 6849, and then one set of panels was deoxidized inDeoxidizer 6/16 and another set was deoxidized in Deoxalume 2310, bothproducts are commercially available from Henkel Corporation. Then, afterthe deoxidizing step, sets of panels were processed in conversioncoating Samples A-D at room temperature (22° C.) for 10 minutes). Thepanels were allowed to air dry at room temperature for 72 hours andexposed to 336 hours ASTM B117 salt spray testing. The results are shownin Table 5 below.

TABLE 5 Conversion Salt Spray Result Panel ID Deoxidizer Coating Panel 1Panel 2 174A1 6/16 Sample 1 150 Pits 125 Pits 174A2 2310 Sample 1 25Pits 15 Pits 174B1 6/16 Sample 2 100 Pits 75 Pits 174B2 2310 Sample 2 0Pits 0 Pits 174C1 6/16 Sample 3 100 Pits 100 Pits 174C2 2310 Sample 390% corrosion 90% corrosion 174D1 6/16 Sample 4 0 Pits 0 Pits 174D2 2310Sample 4 0 Pits 3 Pits 90% corrosion is equal to millions of pitsThe results show (in the attached table) that Sample D, with bothguanidine nitrate and tolyltriazole work better than the control oreither of the additives used alone.

This invention provides a trivalent chromium-containing conversioncoating composition that can be used in conversion coating a variety ofmetals including aluminum and aluminum alloy. It can be used in manyvariations of the processes that are employed in conversion coating ofmetal substrates. While the invention has been described in terms ofspecific embodiments thereof, it will be appreciated that other formscould readily be adapted by one skilled in the art. Accordingly, thescope of the invention is to be considered limited only by the followingaspects.

1. An article of manufacture comprising at least one metal surface having an adherent conversion coating deposited thereon, said coating comprising trivalent chromium; oxygen; carbon; nitrogen; and at least one element “M” selected from the group consisting of titanium, zirconium, hafnium, silicon, aluminum, and boron; said coating being substantially free of hexavalent chromium.
 2. The article of manufacture according to claim 1, wherein the at least one element “M” is selected from the group consisting of titanium, zirconium, hafnium and mixtures thereof
 3. The article of manufacture according to claim 2, wherein the at least one element “M” is selected from the group consisting of titanium, zirconium and mixtures thereof
 4. The article of manufacture according to claim 1, wherein the at least one element “M” comprises zirconium.
 5. The article of manufacture according to claim 4, wherein the coating further comprises fluorine.
 6. The article of manufacture according to claim 1, wherein at least a portion of the coating has a molar ratio of trivalent chromium to the element “M” within a range of 1.27:1 to 43.8:1.
 7. The article of manufacture according to claim 1, wherein the coating further comprises an organic corrosion inhibitor.
 8. The article of manufacture according to claim 7, wherein the organic corrosion inhibitor is present in the coating in a weight ratio of the organic corrosion inhibitor to the element “M” within a range of 0.01:1-6.0:1.
 9. The article of manufacture according to claim 1, wherein the coating further comprises an organic corrosion inhibitor based upon one or more of: a thiazole compound, a triazole compound, a tetrazole compound and an imidazole compound.
 10. The article of manufacture according to claim 9, wherein the organic corrosion inhibitor is a triazole compound.
 11. The article of manufacture according to claim 1, wherein the at least one metal surface is selected from steel, titanium and its alloys, aluminum and its alloys, magnesium and its alloys, zinc and its alloys; aluminum/zinc alloy coated steel and iron/zinc alloy coated steel.
 12. The article of manufacture according to claim 1, wherein the at least one metal surface is selected from at least two of steel, titanium and its alloys, aluminum and its alloys, magnesium and its alloys, zinc and its alloys; aluminum/zinc alloy coated steel and iron/zinc alloy coated steel.
 13. The article of manufacture according to claim 1, wherein the at least one metal surface comprises as least one aluminum or aluminum alloy surface.
 14. The article of manufacture of claim 13, wherein the coated metal surface has corrosion resistance properties characterized by less than 10 pits per 24 square inches when subjected to 5% neutral salt spray at 95° F. ASTM B-117 for at least 96 hours.
 15. The article of manufacture according to claim 1, wherein the coating contains a non-zero amount of hexavalent chromium that is less than 0.04 percent by weight of hexavalent chromium.
 16. The article of manufacture according to claim 1, wherein the dried coating has a total add-on mass of at least 10 milligrams per square foot of metal surface coated.
 17. The article of manufacture according to claim 16, wherein the dried coating has a total add-on mass of no more than 150 milligrams per square foot of metal surface coated.
 18. The article of manufacture according to claim 1, wherein the coating contains no hexavalent chromium. 